When a physical storage apparatus is provided on the basis of a capacity contracted by a user, a technique for virtualizing a physical storage apparatus and providing a disk with a capacity demanded by a user as a virtual disk is used. A user usually creates a virtual disk with an extra capacity.
Thick provisioning is a method of fully allocating a disk capacity demanded by a user to a virtual disk at the time of creation of the virtual disk. Since a demanded capacity is fully allocated to a virtual disk regardless of whether the capacity is to be used, a user's demand for a capacity exceeding the physical capacity of a storage apparatus is not received. On the other hand, thin provisioning is a technique for dynamically allocating a capacity to a virtual disk on the basis of the size of data to be actually written without fully allocating a disk capacity demanded by a user to the virtual disk at the time of creation of the virtual disk. With the thin provisioning technique, an extra capacity that is set by a user but may remain unused is not allocated. Accordingly, a demand from another user who plans to actually use a capacity is received, and a demand for a capacity exceeding the physical capacity of a storage apparatus is received from a user (overcommitment).
There is a migration technique for monitoring the user's use of a virtual disk and moving the virtual disk in a physical disk to another physical disk when it is determined that it is difficult to provide a capacity contracted by a user with the current physical disk. With the migration technique, data in the virtual disk is copied and the copied data is stored in a new virtual disk created in another virtual storage. At that time, the user accesses the new virtual disk with an identifier assigned to the original virtual disk. Methods of selecting a migration target virtual disk with the migration technique include a method of selecting a virtual disk whose amount of practical use is the maximum and a method of selecting a virtual disk whose amount of practical use is small but the increase in the amount of practical use is the maximum.
In a case where a virtual disk creation request is received from a user, a virtual disk is generally created in a virtual storage having the maximum free space. Although it is determined that there is sufficient free space in a virtual storage at the time of creation of a virtual disk, the shortage of free space may occur in accordance with an operational state. In this case, when overcommitment is performed, the migration of some virtual disks to another virtual storage is performed to ensure a disk capacity committed for a user. By performing the migration of a virtual disk whose amount of practical use is large from a virtual storage, large free space is obtained in the virtual storage. However, since data in the virtual disk has to be copied, the migration of the virtual disk takes a lot of time.
In a case where free space in a virtual storage is reduced, the amount of overcommitment has to be reduced to ensure a virtual disk size committed for a user (the amount of overcommitment has to be set to zero when there is no free space in a physical disk). By performing the migration of a virtual disk whose amount of practical use is large, both the acquirement of free space in the virtual storage and the reduction in the amount of overcommitment is achieved. However, in the migration of a virtual disk whose amount of practical use is large, large amounts of data have to be copied. In order to complete the migration within a short time, a large network bandwidth has to be used.
Examples of the related art include Japanese National Publication of International Patent Application No. 2002-533830 and Japanese Laid-open Patent Publication No. 2005-202631.